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The Purpose of the High Voltage Transformer Used in Microwave Ovens and How to Safely Test this High Voltage Component
and from the DVD video You Can Fix Your Microwave Oven
Important Safety Information
Microwave ovens are among the most dangerous appliances to work on. Before attempting any troubleshooting, testing or repairs, for your personal safety, we strongly urge you to carefully read the very important safety precautions found by clicking here and please read our disclaimer at the bottom of this page.
It might be said that the HIGH-VOLTAGE TRANSFORMER is the "muscle" of the microwave oven. With an input of 120 VAC (or 240 VAC in many commercial models) applied to the primary winding, the high-voltage transformer (also referred to as power or plate transformer) steps up that primary voltage to a very high voltage. This high voltage is then boosted even higher by the voltage-doubling action of the capacitor and diode. The resulting voltage, about 3000 - 5000 volts DC (depending on the model), is available at the high voltage (output) tap (see illustration).
In most models, the transformer also incorporates a second output winding, as shown in the illustration below. This is a stepped-down output that provides the filament voltage (typically 3 to 4 VAC) to the magnetron tube. DO NOT attempt to measure this voltage.
Some older models use a separate filament transformer . If that is the case, skip steps 3 through 5.
WARNING! THESE VOLTAGES CAN BE LETHAL! NEVER, EVER ATTEMPT TO MEASURE THE 
OUTPUT VOLTAGES OF THE HIGH-VOLTAGE TRANSFORMER!
How To Test The High-Voltage Transformer Used in Commercial and Residential Microwave Ovens
Microwave ovens are among the most dangerous appliances to work on. Before attempting any troubleshooting, testing or repairs, for your personal safety, we strongly urge you to carefully read the very important safety precautions found by clicking here and please read our disclaimer at the bottom of this page.
- UNPLUG the oven.
- DISCHARGE ALL HIGH-VOLTAGE CAPACITORS. (Procedure)
- Disconnect the high-voltage lead from the transformer's high-voltage terminal (or tap).
- With an ohmmeter set to the lowest resistance scale, R X 1, measure the resistance from the high-voltage terminal to the transformer chassis (ground).
- Some early models are equipped with two or three high-voltage taps. If this is the case, perform the measurement from each tap to chassis ground.
- The meter should read about 55 to 70 ohms, depending on which high-voltage tap is being measured and the model being tested.
- A substantially higher or lower reading would indicate that the transformer is defective.
- Carefully disconnect the leads from the primary (input) terminals
- Measure from one transformer primary terminal to the other for a normal reading of less than 1 ohm (i.e., .22 ohms)
- A substantially higher reading would indicate that the primary winding is open.
- Set the meter to its highest resistance scale and check from each transformer primary terminal to the transformer chassis (ground) for a normal reading of infinity (open circuit).
- Any measurement of resistance would indicate some degree of a short to ground
- Set the meter back to its lowest resistance scale. Then carefully disconnect the filament leads and measure from one filament terminal to the other for a normal resistance of less than 1 ohm.
- Set the meter to its highest scale and measure from each filament terminal to chassis ground for a normal reading of infinity (open circuit).
- Any measurement that is substantially different from the normal readings would indicate a defective transformer.
- Any measurement that is substantially different from the normal readings would indicate a defective transformer.
Illustration from THE COMPLETE MICROWAVE OVEN SERVICE HANDOOK |
If you would like to learn more about repairing commercial and residential microwave ovens and become part of a multi-million dollar service industry, we invite you to take a look at Microtech's powerful new computerized training course.
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 THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK 2009 Operation, Maintenance, Troubleshooting and Repair J. Carlton Gallawa |
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